This invention relates to cementing of liners in wellbores, and more specifically to an improved method of cleaning a cement plug from the top of a liner after it has been cemented in place.
In the drilling of oil and gas wells, it is common to place one or more casing strings in the wellbore extending downward from the surface. These casing strings are typically held in place by cement placed between the borehole wall and the exterior of the casing string(s).
In many cases, a "liner" is run into the uncased portion of a wellbore. A liner is a truncated section of casing that is used to case open hole below a previously set casing string. The liner extends from the bottom of the open hole section and overlaps up into the previously set casing string. The overlap can range from 100 feet to 500 feet. Liners are usually suspended from the previously set casing string by means of a liner hanger/packer assembly. The liner is cemented in place to create a bond between the pipe and the formation. In cementing the liner, typically the cement is pumped down to the liner and through a running-in tool, followed by a displacement fluid that forces the cement into the annulus between the borehole wall and the liner, and into the overlap between the liner and the previously set casing string, and above the running-in tool. After removing the running-in tool, it is common to end up with a cement "plug" in the lower casing above the top of the liner that has to be drilled out before the well can be placed on production. A further complication is that part of the cement plug, before it sets, settles into the top portion of the liner, and must also be drilled out. Normal cleanout practice is to drill out the cement plug above the liner top with a large diameter bit, and then replace the bit with a smaller one and drill out the plug in the top of the liner. This obviously requires a time consuming "round trip" of the drill pipe in order to change the drill bit.